Resins used in certain adhesive applications where small quantities of resin, typically less than 10 percent by weight of the elements, are spread evenly over a large surface area, first are emulsified to provide a ready means of spray-applying the small quantity of resin evenly.
Typical applications of such emulsions are in panel board production, such as waferboard, particle board and plywood production. In waferboard production, about 2 to about 3% by weight of resin typically is applied whereas in particle board production about 6 to about 9% of resin is applied. In plywood production, depending on the application techniques used, the resin emulsions may be further modified by adding fillers and thickeners, such as wheat flour, sawdust, clay and oil.
Thermosetting phenol-formaldehyde resins that contain benzylic ether linkages ortho to the phenolic hydroxyl group are described in Canadian Patent No. 927,041, U.S. Pat. No. 3,485,797 and in our copending application Ser. No. 567,865 filed Apr. 15, 1975.
The latter resins, although generally similar in gross chemical structure, nevertheless are different from each other in many respects. Some of these differences may be detected by comparing the infra-red spectra of the resins and others by comparing their nuclear magnetic resonance spectra. Other differences are demonstrated by their performance characteristics in specific applications.
Resins produced by reacting formaldehyde with phenol in an aqueous reaction medium in the presence of a water-soluble metal carboxylate catalyst therefor, either according to the procedure of Canadian Patent No. 927,041 or according to our copending application Ser. No. 567,865 mentioned above, can be emulsified in water, under appropriate conditions, to give stable dispersions. On the other hand, resins produced by reacting formaldehyde with phenol under anhydrous conditions in accordance with U.S. Pat. No. 3,485,797 do not yield stable emulsions under these conditions.
It has been found that the emulsification characteristics of the resins are dependent on the relative concentrations of phenol and formaldehyde used to form the resin. Thus, resins made with a mole ratio of formaldehyde to phenol of 1.6:1 or less provide stable emulsions at viscosities in excess of 50,000 cps at ambient temperatures of around 24.degree. C and in excess of 9,000 cps at 50.degree. C, while resins formed with formaldehyde to phenol mole ratios in excess of 1.7:1 give stable emulsions even at viscosities as low as 1,000 centipoise at ambient temperatures of about 24.degree. C. (350 centipoise at 50.degree. C).
However, resins made with mole ratios of formaldehyde to phenol less than 1.5:1 do not yield stable emulsions even at very high consistencies corresponding to such high viscosities as 12,000 cps at 50.degree. C.
In the examples of Canadian Patent No. 927,041, it is indicated that for plywood production using an emulsified form of the resin, press times may be decreased by addition to the emulsion of a strong acid, such as para-toluene sulfonic acid. The resin is produced at a mole ratio of formaldehyde to phenol of 1.5:1, which would require the resin to be prepared at high viscosities, in excess of 50,000 centipoise at 24.degree. C, to be emulsifiable. The high viscosity resin must be warmed to around 70.degree. C for emulsification to be carried out.
If the acid catalyst for curing the resin is added to the high viscosity resins prior to emulsification, then, upon heating to the temperature required for emulsification of the resins, cross-linking would tend to occur, increasing further the viscosity or leading to gellation.
Addition of acid to the emulsion of the high viscosity resins leads to the bulk of the acid remaining in the aqueous phase, hence increasing the quantity of acid required for catalysts of the resin, possibly to levels of physical impairment of the elements to which the emulsion is applied.